In spite of numerous advances in development and use of color imaging media, there are a number of conditions in which monochrome imaging media must be used. For example, archival or long-term preservation of images may require that images be stored on a monochrome media. As another example, there can be advantages to compact storage of images, where it is desirable to use a monochrome media for preserving a color image, with accompanying encoded information.
There can be a considerable amount of data associated with an image, where the data concerns the image itself. For example, in printing applications, information about an image can include color separation data for corresponding cyan, magenta, yellow, and black (CMYK) inks or other colorants. Typically, color separations can be stored as separate images on monochrome media, so that each color separation is then stored as a separate monochrome image. For example, U.S. Pat. No. 5,335,082 (Sable) discloses an apparatus using a plurality of monochrome images as separations of a composite color image. Similarly, U.S. Pat. No. 5,606,379 (Williams) discloses a method for storing color images on a monochrome photographic recording medium in which separate R, G, and B or lightness and chroma channels are stored as separate images. Such methods may be acceptable for some types of storage environments; however, it can be appreciated that there would be advantages in storing fewer images and in providing a more compact arrangement.
A number of existing methods for encoding data associated with an image are directed to the problem of encoding color image information within a monochrome image. Examples of solutions for this type of image-data encoding include the following:                U.S. Pat. No. 5,557,430 (Isemura et al.) discloses a method for processing a color image in order to encode color recognition data on a resulting monochrome image. The method described in U.S. Pat. No. 5,557,430 provides some amount of color information available; however, such a method is usable only in limited applications, such as where only a few spot colors are used on a document, such as a business presentation.        U.S. Pat. No. 5,701,401 (Harrington et al.) discloses a method for preserving the color intent of an image when the image is printed on a monochrome printer. Distinctive patterns are applied for each color area.        U.S. Pat. No. 6,179,485 (Harrington) discloses a method for encoding color information in monochromatic format using variously stroked patterns. This method is primarily directed to preserving color intent for fonts and vector (line) drawings.        Similarly, U.S. Pat. No. 6,169,607 (also to Harrington) discloses methods for encoding color data in monochrome text using combinations of bold, outline, and fill pattern effects.        U.S. Pat. Nos. 4,688,031 and 4,703,318 (both to Haggerty) disclose methods for monochromatic representation of color using background and foreground patterns.        
Overall, the methods disclosed in U.S. Pat. Nos. 5,557,430; 5,701,401; 6,179,485; 4,688,031; and 4,703,318 may provide some color encoding that is useful for documents using a very limited color palette, such as business documents and charts. However, these methods would be unworkable for a fall-color image, where the need for a pixel-by-pixel encoding would require considerably greater spatial resolution than these methods provide. At best, such methods may be able to provide a rudimentary approximation of color using relative lightness levels. However, there is no provision in any of the schemes given in the patents listed above for encoding of additional data related to the color image when it is represented in monochrome format.
Known methods used for encoding data associated with an image include that disclosed in U.S. Pat. No. 5,818,966 (Prasad et al.), which discloses encoding color information along a sidebar that prints with a monochrome version of a document. This solution would have only limited value, such as with charts and other business graphics using a palette having a few colors.
Each of the solutions noted above is directed to encoding data about the image itself, such as color data. However, it may be useful to encode other types of data that, although not directly concerned with image representation itself, may be associated with an image. For example, an image can have associated audio data, animation data, measurement data, text, or other data, where it is advantageous to have such data coupled in some manner with the image. Use of a sidebar, such as disclosed in U.S. Pat. No. 5,818,966 provides some solution; however, such a solution requires additional media area that may not be inherently coupled to an image. Because most images are stored in a rectangular format, any additional patch of information must be stored above, below, or on either side of the image. Accompanying information would take up additional space on the media. In addition, any encoded information provided in a separate area of the storage medium could be intentionally or unintentionally separated from the image itself.
Methods for encoding data in visible form on a monochromatic medium include the following:                U.S. Pat. No. 5,091,966 (Bloomberg et al.) discloses the use of monochromatic glyph codes encoded onto a document image, in visual juxtaposition to the image. Notably, the area in which the glyph codes are encoded is separate from the document image itself with this solution.        U.S. Pat. Nos. 6,098,882 (Antognini et al.) and 4,939,354 (Priddy et al.) disclose methods for encoding digital data onto paper in compact form using bi-tonal markings grouped in a spatial array of cells. The ability to provide increasingly more compact data storage on monochrome media, using methods such as those disclosed in U.S. Pat. Nos. 6,098,882 and 4,939,354, can be attributed, in large part, to continuing improvement in the spatial resolution of desktop scanners.        U.S. Pat. No. 5,278,400 (Appel) discloses a method for encoding data in a cell comprising multiple pixels, where the halftone gray level of each individual pixel, in combination with other pixels within the cell, encodes a data value for the cell. The method disclosed in U.S. Pat. No. 5,278,400 also takes advantage of increased spatial resolution of scanners, supplemented by the capability of a scanner to sense gray level at an individual pixel within a cell.        
The methods disclosed in U.S. Pat. Nos. 5,278,400, 6,098,882, and 4,939,354 provide data encoding for compact data storage on a monochrome medium. However, neither these methods nor the methods disclosed in the patents cited above provide a mechanism for integrally coupling data to an associated image. These methods also require space on the monochrome medium, in addition to that required for the image itself.
Some types of monochrome media, such as paper, for example, allow reproduction of only a limited range of perceptible densities. That is, only a few different density levels can be reliably printed or scanned from such types of media. However, there are other types of monochrome media that have pronouncedly greater sensitivity. Conventional black and white photography film, for example, is able to faithfully and controllably reproduce hundreds of different gray levels, each measurably distinct. Other specialized films and photosensitive media have been developed that exhibit wider overall dynamic range and higher degrees of resolvable density, able to produce a higher number of distinct grayscale values.
It is instructive to observe that the term “grayscale” is conventionally associated with a range of densities where the monochromatic color hue is black. However, for the purposes of this application, the monochromatic color hue, or color base, for a grayscale image need not be black, but could be some other color. For example, some types of monochrome film have a very dark blue color hue that could be used as the color base for grayscale imaging. Regardless of the precise color hue, the term “grayscale” as used herein relates to a range of measurable density values of a single base color, formed at individual pixel locations on a digital preservation medium.
It is instructive to note that the human viewer perceives only a limited number of grayscale gradation values, centered on a range that is well within the overall dynamic range of most types of photosensitive media. Generally, a bit depth of 8-bits is sufficient for storing the grayscale values perceptible in monochrome images. While, for human perception, there may be no need for visible representation exceeding a bit depth of 8-bits, it could be possible to reproduce an image having a larger bit depth, with 10, 12, or greater bits of resolution, for example, using photosensitive media described above. In fact, many conventional scanners have additional sensitivity for grayscale resolution. The four-color printing industry, for example, uses high-resolution color scanners that are able to provide very high spatial resolution and very sensitive color resolution. As just one example, the SG-8060P MarkII High-end Input Scanner from Dainippon Screen claims to be capable of scanning at 12,000 dpi and providing 48-bit RGB resolution. Anticipated improvements in scanning technology are expected to make the capability for such high resolution and high density sensitivity more readily accessible and more affordable. This would mean, for example, that a scanner could have sufficient sensitivity to provide data with a bit depth exceeding 8-bits when scanning a highly sensitive media, even though 8-bit grayscale representation is sufficient for storing an image in human-readable form.
Conventionally, in converting a full-color image to a monochrome format only the relative lightness or darkness value of a color is used to determine a corresponding grayscale representation. Chroma information, which indicates color hue content, is largely ignored. For this reason, restoration of original color information to an image, once converted to monochrome format, is not easily feasible. It can be appreciated that image storage solutions that preserved some color information, even if approximate, could be advantageous.
Thus it can be seen that conventional document storage and preservation solutions fall far short of meeting the need to integrally couple data related to an image to the image itself. Even though the capability exists for reproducing and measuring image density sensitivity well in excess of the human-perceptible range, no use has been made of this excess capability for its data storage potential.